This invention relates generally to the manufacture of medical devices, and specifically to the testing of generally flat components of medical devices, e.g., the leaflets of mechanical heart valve prostheses. These valves have a generally rigid frame and at least one generally rigid leaflet operatively attached to the frame so that, in the closed position, the leaflet contacts adjacent leaflets and/or the valve frame thereby closing the valve and preventing the flow of blood therethrough. In one design, two semicircular hinged leaflets pivot within groves or other geometry contained in the orifice housing. In the open position, the leaflets separate from each other, and open radially outward toward the inner walls of a body lumen in which the valve is located. Examples of these designs are disclosed in U.S. Pat. Nos. 4,233,690; 4,689,046; 4,692,165; and 4,863,458; all of which are incorporated herein by reference.
One of the most common materials presently in use for forming these leaflets is Pyrolite.RTM. carbon. In one use, an inner graphite core is surrounded with Pyrolite.RTM. carbon, forming two outer layers. The formed Pyrolite.RTM. carbon pieces may have small flaws that are undetectable without a proof test. The proof test ensures that only the highest quality material is used in this medical device, preventing the chance for mechanical failure during the operational lifetime of the device.
A proof test is necessary to determine whether a planar, generally brittle material, e.g., a Pyrolite.RTM. carbon mechanical heart valve leaflet, contains flaws at or above a critical flaw size which might affect the operational life of the valve. The presence of flaws of such a critical size for Pyrolite.RTM. valve leaflets can be determined by exposing the leaflet to an appropriate stress field.
Prior art proof tests use a pair of mated cylindrical molds called "shoes" that apply a uniaxial stress field across the surface of a planar leaflet. These prior art tests using cylindrical-shaped shoes require four separate orientations on both flat surfaces to adequately proof test the leaflet. The leaflet to be tested is placed between the mated shoes, and a compressive load is placed on the leaflet by pressing the shoes together. The load is then relieved, allowing the shoes to be separated and the leaflet repositioned to measure leaflet stress/strain in another direction. This process is repeated until at least four separate orientations were tested. If the leaflet breaks or cracks, it is discarded. Based on the geometry of the shoes, this process concentrates stress distribution along a single axis, and requires load testing in multiple orientations--a time-consuming process. Also, placement location of the leaflet between the cylindrical shoes is critical, adding further to the complexity and time involved in the test.
The proof test of the present invention improves the stress distribution across the leaflet surface under test, reduces testing time by reducing the need for repositioning the leaflet and generally simplifies flat leaflet proof testing.